Frequency tunable optical oscillator with fiber grating mirrors

ABSTRACT

There is provided a millimeter wave band frequency optical oscillator predicted to be used as a millimeter wave oscillating frequency signal source in a base station of a millimeter wave wireless transmission system. The optical oscillator has a double resonator structure in which a pair of wavelength tunable fiber grating mirrors are inserted into a unilateral fiber-ring laser resonator in order to internally and additionally form a linear laser resonator. The double resonator structure composed of the two stable laser resonators can oscillate laser of two modes. Due to a beat phenomenon occurring between the two modes, received laser is modulated to an ultra-speed frequency of 60 GHz or greater. A variation in the gain within a resonator is induced by a polarization controller using the dependency of laser modes upon polarization. A modulation frequency is consecutively changed from 60 GHz to 80 GHz by controlling the wavelength of light reflected by the fiber grating mirrors.

BACKGROUND OF THE INVENTION

[0001] This application claims the priority of Korean Patent ApplicationNo. 2002-67053, filed on Oct. 31, 2002, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to a high frequency opticaloscillator in the field of wireless telecommunications, and moreparticularly, to a millimeter wave band frequency optical oscillator.

[0004] 2. Description of the Related Art

[0005] Next-generation ultra-speed wireless Internet services cantransmit a large capacity (about 100 Mbps) of information, which is 10times or greater than the information transmitted by existing wirelessInternet service, to wireless subscribers. In order to achieve this,millimeter wave wireless transmission systems that transmit a largecapacity of information have been actively developed. In the millimeterwave wireless transmission systems, a millimeter wave band frequencyoptical oscillator is anticipated to be used as a millimeter waveoscillating frequency signal source in a base station and also as anoptical line network between base stations.

[0006] In the field of millimeter wave wireless transmission systems,optical oscillators using semiconductor high frequency opticalmodulators and optical oscillators using a resonator modulation methodhave been developed up to now. The optical oscillators using opticalmodulators have a frequency band limited to 30 GHz at most, andaccordingly they are not suitable for a frequency band that is beingcurrently studied. Also, the optical oscillators using opticalmodulators are not practical because it is difficult to manufacture anoptical source. On the other hand, the present applicant hascontinuously reported improved versions of the optical oscillators usinga resonator modulation.

[0007] For example, the present applicant has Korean Patent ApplicationNo. 1999-61149 filed on Dec. 23, 1999, which discloses an opticaloscillator capable of being modulated to a maximum of 20 GHz due to abeat phenomenon occurring between two laser modes by oscillating the twolaser modes within a resonator formed by combining a fiber-ring laserresonator and a fiber linear laser resonator using a 50% fiber coupler.The present applicant also has Korean Patent Application No. 2000-54801filed on Sep. 19, 2000, which discloses an oscillator capable of beingmodulated to a maximum frequency of 40 GHz at most by combining twofiber-ring laser resonators using the 50% fiber coupler.

[0008] The optical oscillators proposed by the present applicants areimproved versions of conventional optical oscillators but havecomplicated structures because two individual fiber laser resonatorsmust be combined using a fiber coupler. In addition, in order to be usedas a millimeter wave oscillating frequency signal source, the proposedoptical oscillators require to be modulated to a wider frequency band.

SUMMARY OF THE INVENTION

[0009] The present invention provides a frequency tunable opticaloscillator in which a modulation frequency consecutively varies at ahigh modulation frequency of 60 GHz or greater, which could not beachieved by a currently-used optical oscillator.

[0010] According to an aspect of the present invention, there isprovided a frequency tunable optical oscillator, in which a pair offiber grating mirrors by which the wavelength of light that can bereflected varies are installed within a fiber-ring laser resonator inorder to additionally form a linear laser resonator that reciprocatesbetween the two fiber grating mirrors. Input light can be modulated toan ultra-high frequency by generating a beat phenomenon between twolaser modes oscillated by the fiber-ring laser resonator and the linearlaser oscillator, respectively. A modulation frequency consecutivelyvaries by changing the wavelength of light reflected by the fibergrating mirrors.

[0011] Preferably, the fiber-ring laser resonator includes apolarization controller for modulating the frequency of output light.

[0012] Preferably, the fiber-ring laser resonator includes: a wavelengthdivision multiplexing (WDM) coupler receiving pump laser; a lightamplifying fiber amplifying light received from the WDM coupler; adispersion shifting fiber performing nonlinear polarization on lightreceived from the light amplifying fiber; a direction controllercontrolling the direction of light received from the dispersion shiftingfiber; a polarization controller modulating the frequency of outputlight by controlling the angle of light received from the directioncontroller; and an output port outputting the output light. The fibergrating mirrors are installed before and behind the directioncontroller, respectively.

[0013] Preferably, the length of the light amplifying fiber is 3 m, thelength of the dispersion shifting fiber is 4 m, and the output port is a10% fiber coupler.

[0014] In a single laser resonator, laser is oscillated at a wavelengthhaving the maximum gain during one-time resonance. In a unilateralfiber-ring laser resonator, laser is oscillated at a single wavelengthhaving the maximum gain obtained from the gain of an opticalamplification medium and a gain depending on a selection of apolarization mode made when birefringence is performed on thepolarization mode of laser existing within a fiber. In the presentinvention, a pair of fiber grating mirrors that reflect only lighthaving a particular wavelength are added to the unilateral fiber-ringlaser resonator, thereby internally forming another resonator thatreciprocates between the two mirrors. Consequently, an opticaloscillator according to the present invention generates two independentlaser modes.

[0015] As described above, the present invention provides an opticaloscillator in which a modulation frequency can consecutively vary from60 GHz to 80 GHz. In the optical oscillator according to the presentinvention, a polarization mode for oscillated laser is selected bycontrolling the angle of a polarization controller installed within aresonator, and thus a beat frequency between two laser polarizationmodes can be changed to two frequency ranges, i.e., 64 GHz and 222 GHz.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and other features and advantages of the presentinvention will become more apparent by describing in detail exemplaryembodiments thereof with reference to the attached drawings in which:

[0017]FIG. 1 is a schematic structure diagram of a frequency tunableoptical oscillator with fiber grating mirrors according to the presentinvention;

[0018]FIG. 2 is a graph showing light wavelengths and gains with respectto variations in the angle of a polarization controller in a laserresonator according to the present invention;

[0019]FIG. 3 is a graph showing variations in a modulation frequencywith respect to the angle of a polarization controller in a laserresonator according to the present invention; and

[0020]FIG. 4 is a graph showing variations in the modulation frequencyof an optical oscillator with respect to variations in the wavelength ofreflected light of fiber grating mirrors in a laser resonator accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present invention will bedescribed in detail with reference to the attached drawings. The presentinvention may, however, be embodied in many different forms and shouldnot be construed as being limited to the embodiments set forth herein;rather, these embodiments are provided so that the present disclosurewill be thorough and complete, and will fully convey the concept of theinvention to those skilled in the art.

[0022] Referring to FIG. 1, an optical oscillator using fiber gratingmirrors, according to the present invention, is basically constituted ofa single fiber-ring laser resonator 100. In the optical oscillator, apair of fiber grating mirrors 110 a and 110 b, in which the wavelengthof light that can be reflected varies, are installed within thefiber-ring laser resonator 100 such that a linear laser resonator 120which reciprocates between the two fiber grating mirrors 110 a and 110 bis added to the inside of the optical oscillator. Due to a beatphenomenon between two laser modes (first and second modes) oscillatedby the two laser resonators 100 and 120, output light is modulated to anultra-high frequency (60 GHz or greater). By varying the wavelength oflight that can be reflected by the fiber grating mirrors 110 a and 110b, a high performance laser source capable of consecutively varying amodulation frequency can be implemented.

[0023] The ring laser resonator 100 includes a wavelength coupler 125for receiving pump light, a 3 m-long light amplifying fiber (LAF) 130, a4 m-long dispersion shifting fiber (DSF) 135, a polarization controller(PC) 140, a direction controller (DC) 145 which blocks a light path, anda 10% fiber coupler (FC) 150 for output light. An example of the LAF 130is an EDFA that can amplify a wavelength of 1550 nm by doping a fiberwith Er³⁺ ions. In this structure, the ring laser resonator 100 advanceslight in one direction. The fiber grating mirrors 110 a and 110 b areinstalled in front of and behind the DC 145.

[0024] The wavelength coupler 125 is installed at one end of the LAF 130in order to receive pump light. For example, the wavelength coupler 125is a wavelength division multiplexing (WDM) coupler that receives pumplaser composed of a laser diode (LD) with a 980 nm wavelength. The LDhaving a 980 nm wavelength can be made of AlInGaAs. Light received viathe wavelength coupler 125 is amplified by the LAF 130. The DSF 135performs a non-linear polarization on light transmitted by the LAF 130.Light generated over a wide frequency band due to pumping is subjectedto birefringence in the DSF 135 and accordingly has a gain depending onpolarization. The DC145 controls the direction of light received fromthe DSF 135 so that the light has a constant oscillation direction. ThePC 140 and a linear polarizer 142 control the orientation angle andphase retardation of light received from the DC 145 and, consequently,are used to select only a polarization state where resonance ispossible. The FC 150 serves as an output port for outputting outputlight. Here, the output light has a 1530 nm wavelength.

[0025] The fiber grating mirrors 110 a and 110 b installed before andbehind the DC 145 correspond to wavelength selection mirrors. In thewavelength selection mirrors, wherein the refractive index of a lightpassage portion at the centre of a fiber is periodically changed, lightwith a particular wavelength existing within a narrow wavelength rangedefined by the refractive index changing period is reflected, whilelight existing beyond the defined wavelength range passes. Due to theinstallation of the fiber grating mirrors 110 a and 110 b, the linearlaser resonator 120 is formed in which only a particular wavelengthreciprocates between the two fiber grating mirrors 110 a and 110 b.Accordingly, a double resonator structure is formed. In the doubleresonator structure, since the ring laser resonator 100 and the linearlaser resonator 120 composed of the fiber grating mirrors 110 a and 110b provide different conditions, independent laser light beams can begenerated from the single optical oscillator. Hence, the ring laserresonator 100 can be transformed into a double-mode laser oscillator. Iftwo laser modes are oscillated at the same time, optical power ismodulated and oscillated to a beat frequency generated due to thedifference in frequency between the two modes.

[0026] However, since the two laser resonators 100 and 120 must dividethe gain of an optical amplification medium, one of the wavelengths andpolarizations of the fiber grating mirrors 110 a and 110 b must be veryappropriately selected. In addition, it is technically difficult thatthe two laser resonators 100 and 120 divide the gain of an opticalamplification medium depending on an irregular birefringence occurringwithin a fiber. Since the optical oscillator according to the presentinvention is designed to be able to simultaneously oscillate lasers oftwo modes using a birefringence, the optical oscillator has a simplestructure and can oscillate high frequency laser with a frequency of 60GHz or greater that cannot be achieved by existing optical modulators.

[0027] The range of the wavelength of light that can be reflected byeach of the fiber grating mirrors 110 a and 110 b can vary bymechanically (or electrically) adjusting the grating interval of each ofthe two mirrors 110 a and 110 b. For example, the grating of each of thefiber grating mirrors 110 a and 110 b is strained by a mechanical stressapplied to both sides of the mirror, and consequently the wavelengthrange of light that can be reflected is changed. The wavelength range ofreflected light can also be changed by heating each of the fiber gratingmirrors 110 a and 110 b. The variations in the wavelength of lightreflected by the fiber grating mirrors 110 a and 110 b affect a changein the wavelength of oscillated laser. Consequently, if the wavelengthof light that can be reflected is consecutively changed, a modulationfrequency may be changed consecutively, e.g., from 60 GHz to 80 GHz.

[0028] Compared to the aforementioned complicate optical oscillatorsfiled in 1999 and 2000 by the present applicant, the optical oscillatoraccording to the present invention does not need to use a fiber couplerfor coupling a ring laser resonator to a linear laser resonator orcoupling two ring laser resonators. Therefore, the optical oscillatoraccording to the present invention can overcome an oscillation at a lowfrequency due to a loss caused by the fiber coupler.

[0029] Also, the optical oscillator according to the present inventionrequire neither a second laser resonator (i.e., a linear or ring laserresonator) to be coupled to the basic ring laser resonator nor a PC tobe installed on the second laser resonator, such that the opticaloscillator according to the present invention has a simple structure.Furthermore, the optical oscillator according to the present inventioncan vary the modulation frequency through a simple mechanicalmanipulation to control the grating interval of a fiber grating mirror,thereby providing a high performance.

[0030] The structure of the optical oscillator according to the presentinvention of FIG. 1 will now be described in more detail with referenceto FIGS. 2 through 4. FIG. 2 is a graph showing light wavelengths andgains with respect to variations in the angle of the PC 140 of theoptical oscillator of FIG. 1. FIG. 3 is a graph showing variations inthe modulation frequency of laser with respect to the angle of the PC140 in the optical oscillator of FIG. 1. FIG. 4 is a graph showingvariations in the modulation frequency of the optical oscillator of FIG.1 with respect to variations in the wavelength of reflected light of thefiber grating mirrors 110 a and 110 b.

[0031] Light beams of a plurality of wavelengths produced due tospontaneous emission of electrons pumped by a pump laser (not shown)proceed along a fiber at both ends of the LAF 130. Light beams beyondthe wavelength range of light reflected by the fiber grating mirrors 110a and 110 b pass the DC 145 counterclockwise as shown in the first modeof FIG. 1 and travel within the ring laser resonator 100 whose length is7 m (the sum of the 3 m length of the LAF 130 and the 4 m length of theDSF 135). At this time, laser of the first mode is oscillated inaccordance with the integrated gain of a polarization gain correspondingto a birefringence by the ring laser resonator 100 and a pumping gain ofthe LAF 130 as shown in FIG. 2. On the other hand, light beams existingwithin the wavelength range of light reflected by the fiber gratingmirror 110 b are reflected by the fiber grating mirror 110 b before theDC 145 and pass the LAF 130 again and are reflected by the fiber gratingmirror 110 a behind the DC 145, such that the light beams reciprocatebetween the two fiber grating mirrors 110 a and 110 b. In this way,laser of the second mode is generated by travelling the path of thelinear laser resonator 120 composed of the two fiber-grating mirrors 110a and 110 b, which is different from the resonator path travelled by thefirst mode laser.

[0032] The laser of the second mode undergoes a birefringence of a fiberwhile reciprocating and provides a loss of an output port twice as greatas the output port loss of the first mode laser. However, the secondmode laser provides a doubled optical amplification gain while going toand returning from a 14 m-long resonator, the number 14 m being nearlytwice the travelling distance of the first mode laser. Hence, since thesecond mode laser has a higher gain than the gain of the first modelaser travelling a simple path, the second mode laser may be oftenoscillated alone in unless the high gain is controlled by the PC 140.

[0033] If the PC 140 and the wavelength range of reflected light areappropriately controlled, the first and second modes can besimultaneously oscillated. The gain of a polarization mode within acertain range was simulated as shown in FIG. 2.

[0034] With the central wavelength of light reflected by the fibergrating mirrors 110 a and 110 b being fixed to 1532.64 nm, if the angleof the PC 140 is oriented, the integrated gain of the gains of the firstand second modes greatly varies. Referring to FIG. 2, the wavelength ofa consecutive wave in the oscillating first mode varies as indicated bytwo-dot one-dashed line (a), and the gain of the oscillating first modevaries as indicated by solid line (b). Meanwhile, because the secondmode going to and returning from the linear laser resonator 120 is fixedto a single wavelength, the gain of the second mode is restricted asindicated by dashed line (c), and consequently, an angle area wherelaser is not oscillated may be generated.

[0035] A modulation frequency based on the difference in frequencybetween the two first and second modes was simulated as shown in solidline (1) of FIG. 3. It can be seen from solid line (1) that threemodulation frequencies of 64 GHz, 126 GHz, and 222 GHz can be achieved.However, considering areas where the second mode is not oscillated asshown in dashed line (2), 222 GHz and 64 GHz are achieved at a narrowangle of about 30 degrees. However, because 222 GHz is unstable, itcannot be used.

[0036] The wavelength range of the second mode is determined by thegrating interval of each of the fiber grating mirrors 110 a and 110 b.If the grating interval is mechanically adjusted, the central wavelengthof light reflected by each of the fiber grating mirrors 110 a and 110 bcan be changed by a maximum of about 4 nm. The modulation frequency ofthe optical oscillator according to the present invention with respectto the central wavelength of the fiber grating mirrors 110 a and 110 bthat changes as described above varied as shown in FIG. 4. In FIG. 4, Tdenotes an angle controlled by the PC 140.

[0037] The present invention can provide a frequency tuneable opticaloscillator in which the modulation frequency of generated laserconsecutively varies from 60 GHz to 80 GHz by controlling the wavelengthof light reflected by fiber grating mirrors included in a signal opticaloscillator. The frequency tuneable optical oscillator will be used as afrequency oscillator or a high frequency optical signal oscillator inwired/wireless integrated millimeter wave telecommunication equipmentfor ultra-speed wireless Internet services. In addition, if thefrequency tuneable optical oscillator is utilized as a core componentpart of a wired ultra-speed optical transmission system, it can replacean imported oscillator, thus reducing the manufacturing costs.

[0038] While the present invention has been particularly shown anddescribed with reference to exemplary embodiments thereof, it will beunderstood by those of ordinary skill in the art that various changes inform and details may be made therein without departing from the spiritand scope of the present invention as defined by the following claims.

What is claimed is:
 1. A frequency tunable optical oscillator in which a pair of fiber grating mirrors by which the wavelength of light that can be reflected varies are installed within a fiber-ring laser resonator in order to additionally form a linear laser resonator that reciprocates between the two fiber grating mirrors, a beat phenomenon occurs between two laser modes oscillated by the fiber-ring laser resonator and the linear laser oscillator, respectively, and a modulation frequency consecutively varies by changing the wavelength of light reflected by the fiber grating mirrors.
 2. The frequency tunable optical oscillator of claim 1, wherein the fiber-ring laser resonator includes a polarization controller for modulating the frequency of output light.
 3. The frequency tunable optical oscillator of claim 1, wherein the fiber-ring laser resonator comprises: a wavelength division multiplexing (WDM) coupler receiving pump laser; a light amplifying fiber amplifying light received from the WDM coupler; a dispersion shifting fiber performing nonlinear polarization on light received from the light amplifying fiber; a direction controller controlling the direction of light received from the dispersion shifting fiber; a polarization controller modulating the frequency of output light by controlling the angle of light received from the direction controller; and an output port outputting the output light, wherein the fiber grating mirrors are installed before and behind the direction controller, respectively.
 4. The frequency tunable optical oscillator of claim 3, wherein the length of the light amplifying fiber is 3 m, the length of the dispersion shifting fiber is 4 m, and the output port is a 10% fiber coupler.
 5. The frequency tunable optical oscillator of claim 2, wherein the frequency of the output light is consecutively changed from 60 GHz to 80 GHz by controlling the angle of the polarization controller and the wavelength of light reflected by the fiber grating mirrors. 